[Math] Picking a Pair from 2 Colored Groups

combinatoricsprobability

A box contains three pairs of blue gloves and two pairs of green gloves. Each pair consists of a left-hand glove and a right-hand glove. Each of the gloves is separate from its mate and thoroughly mixed together with the others in the box. If three gloves are randomly selected from the box, what is the probability that a matched set (i.e., a left- and right-hand glove of the same color) will be among the three gloves selected?

Approach:

Since I have 5 pairs of gloves, I pick one ${5 \choose 1}$. Now I have to pick a color from 2 available, Blue or Green ${2 \choose 1}$. From the paired, colored gloves I just picked, I have to 3 on the Blue side and 2 on the Green side but since the 3 on the Blue side are identical, same for the 2 on the Green side, I pick one of the pairs and pick 2 of them ${2 \choose 2}$. For the last glove I can pick any glove. So I can pick out of the 4 remaining ${4 \choose 1}$, and since it doesn't matter what color, I pick one from that pair ${2 \choose 1}$. Hence,
$${5 \choose 1}{2 \choose 1}{2 \choose 2}{4 \choose 1}{2 \choose 1}$$
divide this by ${10 \choose 3}$. This is however not the answer. What am I doing wrong?

Best Answer

We probably need to divide into cases. The following "grinding things out" way is not elegant, but it works. There are $4$ cases, a lot, but they come in very similar pairs.

We count the number of "good" choices of $3$ gloves.

(i) Three green. There are $\binom{4}{3}$ ways to do this, all good, since all such choices lead to a matching pair.

(ii) Three red. There are $\binom{6}{3}$ ways to choose $3$ red. Of them, all but $2$ (all left and all right) are good. So the number of good of this type is $\binom{6}{3}-2$.

(iii) Two green, one red. We must pick one left green and one right green, which can be done in $\binom{2}{1}\binom{2}{1}$ ways. For each such way, we can pick any red. That gives $\binom{2}{1}\binom{2}{1}\binom{6}{1}$ good.

(iv) Two red, one green. The same reasoning as the one above gives $\binom{3}{1}\binom{3}{1}\binom{4}{1}$.

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